1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine having a particulate filter in an exhaust passage. Specifically, the present invention relates to temperature increasing control of a particulate filter during regeneration of the particulate filter.
2. Description of Related Art
A known exhaust gas purification system includes a particulate filter (a diesel particulate filter: DPF) for collecting particulate matters (PM) discharged from a diesel engine. The system increases temperature of the DPF, for instance, over 600° C., when a quantity of the particulate matters deposited on the DPF (a PM deposition quantity) reaches a predetermined value. Thus, the particulate matters deposited on the DFF are combusted and eliminated, and the DPF is regenerated.
At that time, a post-injection, retardation of fuel injection timing, restriction of intake air and the like are usually used as means for increasing the temperature of the DPF. However, deterioration in a fuel cost accompanies the above temperature-increasing means. A combustion speed of the particulate matters increases as the temperature increases. Therefore, the regeneration is finished in a shorter period and the deterioration in the fuel cost due to the regeneration of the DPF is reduced as the temperature increases however, the particulate matters are combusted rapidly and the DPF temperature increases rapidly if the DPF temperature is too high. In such a case, there is a possibility that the DPF is damaged or an oxidation catalyst supported by the DPF is degraded. In order to inhibit the deterioration in the fuel cost and to regenerate the DPF safely, temperature control for maintaining the DPF temperature near target temperature suitable for the regeneration is necessary.
Temperature increasing ability of the temperature increasing means has a limitation and varies in accordance with operating states. Therefore, the DPF temperature fluctuates during the regeneration. In operating states such as a low load operation or deceleration operation, the temperature increasing ability becomes insufficient. Accordingly, a sufficient temperature increasing effect cannot be obtained and the DPF temperature decreases largely. Therefore, in order to inhibit the fluctuation of the DPF temperature during the regeneration, an operation amount of the temperature increasing means should be corrected so that the DPF temperature quickly returns to proximity of target temperature.
In exhaust gas temperature feedback control disclosed in JP-A-2003-172185 (Patent Document 1), the operation amount of the temperature increasing means is corrected with a correction value obtained by multiplying a deviation between a predetermined target temperature and exhaust gas temperature sensed by a sensor and the like or an integration value of the deviation by a predetermined feedback gain (F/B gain). Thus, the DPF temperature is maintained near the target temperature suitable for the regeneration.
Generally, a period for the exhaust gas temperature to reach the target temperature THTRG shortens and response of a control system is improved as the F/B gain is increased. However, the temperature vibrates near the target temperature THTRG and stability is deteriorated as the F/B gain is increased. The response is deteriorated and the stability is improved as the F/B gain is decreased. Therefore, the temperature control should be performed by selecting the optimum F/B gain capable of achieving the response and the stability at the same time.
However, the change in the exhaust gas temperature TH delays with respect to the operation amount of the temperature increasing means mainly due to a delay in heat transfer between a base material of the DPF and the exhaust gas. Moreover, the delay in the control object changes due to a change in the operating state. Accordingly, the optimum F/B gain varies as shown by a solid line “a” in FIG. 3A. The solid line “a” in FIG. 3A represents the optimum F/B gain. Therefore, if the F/B gain is a constant value D as shown in FIG. 3A, overshoot increases and the stability is deteriorated as shown by a broken line B in FIG. 3B in an operating state in which the delay is a large value B. In another operating state in which the delay is a small value C, the response is deteriorated as shown by a chained line C in FIG. 3B. In FIG. 3A, the response is improved and the stability is deteriorated along a direction I. In FIG. 3A, the response is deteriorated and the stability is improved along a direction II.